In recent years, in connection with preservation of global environment and effective utilization of energy intended for resource saving, much attention has been paid to midnight power storage systems, household distributive power storage systems based on photovoltaic technologies, and power storage systems for electric cars.
A first requirement for these power storage systems is that cells used have a high energy density. Vigorous effort has been made to develop lithium ion cells, which are a major candidate for high energy-density cells that meet the first requirement.
A second requirement is that output characteristics are excellent. For example, in a combination of a high-efficiency engine and a power storage system (for example, a hybrid electric car) or a fuel cell and a power storage system (for example, a fuel-cell electric car), the high-power discharge characteristics of the power storage system are required during acceleration.
Electric double-layer capacitors (hereinafter also simply referred to as “capacitors”) that use activated carbon for electrodes have been developed as high-power storage devices. These electric double-layer capacitors have high durability (cycle characteristics and high-temperature storage characteristics) and output characteristics corresponding to about 0.5 kW/L to about 1 kW/L. The electric double-layer capacitor has been considered to be the optimum device for the field in which the above-described high power is required. However, the electric double-layer capacitor has an energy density of only about 1 Wh/L to about 5 Wh/L. Thus, the output duration of the electric double-layer capacitor hinders this capacitor from being put to practical use.
On the other hand, nickel hydrogen cells, currently adopted for hybrid electric cars, offer high power equivalent to that provided by the electric double-layer capacitor and has an energy density of about 160 Wh/L. However, vigorous work is proceeding to further improve the energy density and output power of the nickel hydrogen cell and to make the nickel hydrogen cell more stable at high temperature to improve the durability thereof.
Furthermore, work is proceeding to improve the output power of lithium ion cells. For example, a lithium ion cell has been developed which offers a high power of higher than 3 kW/L at a depth of discharge (a value indicating the percentage of the discharge capacitance of the device at which the device has discharged) of 50%. However, this lithium ion cell has an energy density of 100 Wh/L or less and is designed so as to intentionally hinder the high energy density which is the greatest characteristic of lithium ion cells. Furthermore, the durability (cycle characteristics and high-temperature storage characteristics) of the lithium ion cell is inferior to that of the electric double-layer capacitor. Thus, to allow the lithium ion cell to offer practical durability, the depth of discharge of the lithium ion cell needs to be limited to within the range from 0% to 100%. This further reduces the actually available capacitance, and thus vigorous effort is being made to further improve the durability.
As described above, practical application of storage elements offering high power, a high energy density, and high durability have been demanded. At present, many attempts are being made to develop storage elements called lithium ion capacitors and expected to meet these technical requirements.
The energy density of the capacitor is proportional to the capacitance and withstand voltage thereof. The electric double-layer capacitor has a withstand voltage of about 2 V to about 3 V. Attempts have been made to improve the withstand voltage by using a nonaqueous electrolytic solution containing lithium salt. For example, a capacitor has been proposed which uses activated carbon for a positive electrode and a negative electrode and which also uses a nonaqueous electrolytic solution containing lithium salt (see Patent Documents 1, 2, and 3). However, in this case, the activated carbon in the negative electrode exhibits a low charging and discharging efficiency for lithium ions. Thus, this capacitor has a cycle characteristic problem to be solved. Furthermore, attempts have been made to develop a capacitor in which activated carbon is used for the positive electrode and in which a carbonaceous material such as graphite is used for the negative electrode (see Patent Documents 4, 5, and 6). However, in this case, the graphite in the negative electrode exhibits improper I/O characteristics, and thus when cycle tests are carried out, lithium dendrites are likely to be generated. Hence, this capacitor also has a cycle characteristic problem to be solved.
Moreover, a storage element has been proposed which includes a positive electrode composed of a hydrocarbon material that is a carbonaceous material having a hydrogen/carbon atomic ratio of 0.05 to 0.5 and a porous structure with a BET specific surface area of 300 m2/g to 2,000 m2/g, a BJH mesopore volume of 0.02 ml/g to 0.3 ml/g, and an MP total pore volume of 0.3 ml/g to 1.0 ml/g (see Patent Document 7). The present inventors have carried out additional tests to find that this storage element has a large electrostatic capacitance but disadvantageously offers insufficient output characteristics.
On the other hand, a negative electrode material for storage elements has been proposed in which when a carbonaceous material is attached to the surface of activated carbon as a negative electrode material that stores and emits lithium ions and when a mesopore volume relating to pores with a diameter of 20 angstrom or more and 500 angstrom or less is defined as Vm1 (cc/g) and a micropore volume relating to pores with a diameter of less than 20 angstrom is defined as Vm2 (cc/g), 0.01≦Vm1≦0.20 and 0.01≦Vm2≦0.40 (see Patent Document 8). This negative electrode material exhibits a high charging and discharging efficiency for lithium ions and is thus excellent in output characteristics.
Patent Document 1: JP 11-121285 A
Patent Document 2: JP 11-297578 A
Patent Document 3: JP 2000-124081 A
Patent Document 4: JP 60-182670 A
Patent Document 5: JP 08-107048 A
Patent Document 6: JP 10-027733 A
Patent Document 7: JP 2005-93778 A
Patent Document 8: JP 2003-346801 A